vector - Removing a comma from a c++ output

Question

I wrote this program that sorts numbers in vectors from greatest to least, it works great but the only thing that is bugging me is trying to remove the comma from the last number. Heres my code

#include <iostream>
#include <vector>
#include <cstdlib>
#include <algorithm>
using namespace std;

int main() {

    vector<int> vi1, vi2, vi3;
    srand(987);

    for (int i = 0; i < 10; ++i) vi1.push_back(rand() % 10);
    sort(vi1.begin(), vi1.end());

    for (int i = 0; i < 10; ++i) vi2.push_back(rand() % 10);
    sort(vi2.begin(), vi2.end())

    while(!vi1.empty() && !vi2.empty()) {
        if(vi1.back()>=vi2.back()) {
            vi3.push_back(vi1.back());
            vi1.pop_back();
        }
        else {
            vi3.push_back(vi2.back());
            vi2.pop_back();
        }
    }

    while(!vi1.empty()) {
        vi3.push_back(vi1.back());
        vi1.pop_back();
    }
    while(!vi2.empty()) {
        vi3.push_back(vi2.back());
        vi2.pop_back();
    }

    for (auto i = vi3.begin(); i != vi3.end(); ++i)
        cout << *i << ", ";
    cout << "
Bye..." << endl;
    return 0;
}

and here is the output

9, 9, 9, 8, 8, 8, 6, 6, 3, 3, 3, 3, 2, 2, 2, 1, 1, 1, 0, 0, 
Bye...

as you can see after the last 0 there is a comma which doesn't make sense grammatically speaking. How can I remove that comma?

Answer 1

Rather than removing the simplest way is just to print commas only for other values than the first:

for (auto i = vi3.begin(); i != vi3.end(); ++i) {
    if(i != vi3.begin()) {
        cout << ", ";
    }
    cout << *i;
}

I am using this idiom regularly to format e.g. SQL parameter lists, or similar where a delimiter isn't wanted after the last element but for any others.
There are other ways to detect the first element (e.g. using a bool variable initialized to true before the loop starts, and set to false upon the first iteration).
For your example it seems just checking for vi3.begin() is the easiest and most natural way.

Here's the generic variant in pseudo code:

bool isFirstOutput = true;
for_each(element in list) {
    if(not isFirstOutput) {
        print delimiter;
    }
    print element;
    isFirstOutput = false;
}